rlox/src/interpreter/eval.rs

use std::fmt::Display;

use crate::error::EvalError;
use crate::parser::expr::{BinaryOp, Expr, Literal, UnaryOp};

pub type EvalResult = Result<Primitive, EvalError>;

/*====================================================================================================================*/

pub fn evaluate(expr: Expr) -> EvalResult {
    expr.eval()
}

/*====================================================================================================================*/

#[derive(Debug, PartialEq)]
pub enum Primitive {
    String(String),
    Number(f64),
    Bool(bool),
    Nil,
}

impl Primitive {
    fn is_truthy(&self) -> bool {
        match self {
            Primitive::Bool(false) | Primitive::Nil => false,
            _ => true,
        }
    }
}

impl Display for Primitive {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Primitive::String(s) => write!(f, "\"{s}\""),
            Primitive::Number(num) => write!(f, "{num}"),
            Primitive::Bool(b) => write!(f, "{b}"),
            Primitive::Nil => write!(f, "nil"),
        }
    }
}

/*====================================================================================================================*/

trait Eval {
    fn eval(self) -> EvalResult;
}

impl Eval for Literal {
    fn eval(self) -> EvalResult {
        match self {
            Literal::String(s) => Ok(Primitive::String(s)),
            Literal::Number(num) => Ok(Primitive::Number(num)),
            Literal::Bool(b) => Ok(Primitive::Bool(b)),
            Literal::Nil => Ok(Primitive::Nil),
        }
    }
}

impl Eval for Expr {
    fn eval(self) -> EvalResult {
        match self {
            Expr::Literal(literal) => literal.eval(),
            Expr::Unary(op, expr) => {
                let arg = expr.eval()?;

                match (op, arg) {
                    (UnaryOp::Negate, Primitive::Number(num)) => Ok(Primitive::Number(-num)),
                    (UnaryOp::Not, Primitive::Bool(b)) => Ok(Primitive::Bool(!b)),
                    (UnaryOp::Not, primitive) => Ok(Primitive::Bool(!primitive.is_truthy())),
                    (op, arg) => Err(EvalError::UnaryOpInvalidArgument { op, arg }),
                }
            }
            Expr::Binary(left_expr, op, right_expr) => {
                use Primitive::{Bool, Number, String};

                let left = left_expr.eval()?;
                let right = right_expr.eval()?;

                match (left, op, right) {
                    (Number(left), BinaryOp::Add, Number(right)) => Ok(Number(left + right)),
                    (Number(left), BinaryOp::Subtract, Number(right)) => Ok(Number(left - right)),
                    (Number(left), BinaryOp::Multiply, Number(right)) => Ok(Number(left * right)),
                    (Number(left), BinaryOp::Divide, Number(right)) => {
                        if right == 0.0 {
                            return Err(EvalError::DivisionByZero);
                        }
                        Ok(Number(left / right))
                    }

                    (String(left), BinaryOp::Add, String(right)) => Ok(String(left + &right)),

                    (left, BinaryOp::Equal, right) => Ok(Bool(left == right)),
                    (left, BinaryOp::NotEqual, right) => Ok(Bool(left != right)),

                    (Number(left), BinaryOp::Less, Number(right)) => Ok(Bool(left < right)),
                    (Number(left), BinaryOp::LessEqual, Number(right)) => Ok(Bool(left <= right)),
                    (Number(left), BinaryOp::Greater, Number(right)) => Ok(Bool(left > right)),
                    (Number(left), BinaryOp::GreaterEqual, Number(right)) => Ok(Bool(left >= right)),

                    (String(left), BinaryOp::Less, String(right)) => Ok(Bool(left < right)),
                    (String(left), BinaryOp::LessEqual, String(right)) => Ok(Bool(left <= right)),
                    (String(left), BinaryOp::Greater, String(right)) => Ok(Bool(left > right)),
                    (String(left), BinaryOp::GreaterEqual, String(right)) => Ok(Bool(left >= right)),

                    (left, op, right) => Err(EvalError::BinaryOpInvalidArguments { left, op, right }),
                }
            }
            Expr::Grouping(expr) => expr.eval(),
        }
    }
}